1. Field of the Invention
The present invention relates to a method for inspecting a flexible printed circuit.
The present application is based on Japanese Patent Application No. Hei. 10-221970, which is incorporated herein by reference.
2. Description of the Related Art
Flexible printed circuits (FPC) are used in various electric or electronic appliances. Each of the flexible printed circuits is constituted by an electrically insulating film of a polyimide resin or the like, and wiring patterns of metal foil of copper or the like formed on the electrically insulating film. Such a flexible printed circuit can be provided in a narrow space because it is flexible and thin. Accordingly, reduction in size and thickness of the appliances can be attained.
With the requirement of reducing the size and increasing the degree of integration in electric or electronic appliances, the inter-wiring pitch in the flexible printed circuit has been also narrowed in recent years. Accordingly, when elution of metal ions (ion migration) from a wiring material in the flexible printed circuit occurs, there is a tendency that inter-wiring short-circuiting occurs easily. For example, the ion migration causes the case where the life is shortened to 100 hours in a test under a condition of a temperature of 85.degree. C. and a humidity of 85%.
Accordingly, flexible printed circuits being short-lived because of ion migration are required to be removed as defective products in an inspection stage before shipping. In the short-lived flexible printed circuits, it has been found that foreign matters such as ion impurities, fiber, etc. are present between wiring portions. Accordingly, defective products can be detected when inter-wiring foreign matters in each of the flexible printed circuits are detected by using a microscope.
It is, however, very inefficient to detect inter-wiring foreign matters in the flexible printed circuits individually by microscopic observation. This method is impracticable as an inspection method before shipping.
A method for performing inspection efficiently to detect short-lived flexible printed circuits before shipping has been, therefore, required.